First-order Phase Boundary Research Articles

Inhomogenuous instabilities at large chemical potential in a rainbow-ladder QCD model

In this work we continue our efforts to study the existence of a phase with an inhomogeneous, i.e., spatially varying, chiral condensate in QCD. To this end we employ a previously established method of stability analysis of the two-particle irreducible effective action in a truncation that corresponds to a rainbow-ladder approximation of the quark-gluon interaction of QCD. If the analysis is restricted to homogeneous phases, the phase diagram features a first-order chiral transition in the lower-temperature regime. Performing the stability analysis along the lower-chemical-potential border of the corresponding spinodal region, we find that below a certain temperature the homogeneous chirally symmetric solution is unstable against inhomogeneous condensation. We argue that this instability may persist to chemical potentials above the homogeneous first-order phase boundary, in which case it signals the existence of an inhomogeneous ground state. Our methodology is also applicable for more sophisticated truncations of the QCD effective action. Published by the American Physical Society 2024

Multitasking via baseline control in recurrent neural networks

Changes in behavioral state, such as arousal and movements, strongly affect neural activity in sensory areas, and can be modeled as long-range projections regulating the mean and variance of baseline input currents. What are the computational benefits of these baseline modulations? We investigate this question within a brain-inspired framework for reservoir computing, where we vary the quenched baseline inputs to a recurrent neural network with random couplings. We found that baseline modulations control the dynamical phase of the reservoir network, unlocking a vast repertoire of network phases. We uncovered a number of bistable phases exhibiting the simultaneous coexistence of fixed points and chaos, of two fixed points, and of weak and strong chaos. We identified several phenomena, including noise-driven enhancement of chaos and ergodicity breaking; neural hysteresis, whereby transitions across a phase boundary retain the memory of the preceding phase. In each bistable phase, the reservoir performs a different binary decision-making task. Fast switching between different tasks can be controlled by adjusting the baseline input mean and variance. Moreover, we found that the reservoir network achieves optimal memory performance at any first-order phase boundary. In summary, baseline control enables multitasking without any optimization of the network couplings, opening directions for brain-inspired artificial intelligence and providing an interpretation for the ubiquitously observed behavioral modulations of cortical activity.

Collective Excitation in High-Energy Nuclear Collisions—In Memory of Professor Lianshou Liu

We celebrate the legacies of our friend and mentor Professor Lianshou Liu who was one of the pioneers for the phenomenology of multi-particle interactions and initiated the physics of relativistic heavy-ion collisions in China. In this article, we discuss some of the recent exciting experimental observations on the collective phenomena including collectivity, chirality, criticality, strangeness production, and thermal equilibrium in high-energy nuclear collisions. Future directions, especially the physics at high baryon density, will be discussed with a focus on the first-order phase boundary and hyperon–nucleon interactions.

Thermodynamics of high-pressure ice phases explored with atomistic simulations

Most experimentally known high-pressure ice phases have a body-centred cubic (bcc) oxygen lattice. Our large-scale molecular-dynamics simulations with a machine-learning potential indicate that, amongst these bcc ice phases, ices VII, VII′ and X are the same thermodynamic phase under different conditions, whereas superionic ice VII″ has a first-order phase boundary with ice VII′. Moreover, at about 300 GPa, the transformation between ice X and the Pbcm phase has a sharp structural change but no apparent activation barrier, whilst at higher pressures the barrier gradually increases. Our study thus clarifies the phase behaviour of the high-pressure ices and reveals peculiar solid–solid transition mechanisms not known in other systems.

Phase diagrams of confined square lattice linked polygons.

The phase diagrams of two models of two confined and dense two-dimensional ring polymers are examined numerically. The ring polymers are modeled by square lattice polygons in a square cavity and are placed to be either unlinked or linked in the plane. The phase diagrams of the two models are found to be a function of the placement of the ring polymers and include multicritical points where first-order and continuous phase boundaries meet. We estimate numerically the critical exponents associated with the phase boundaries and the multicritical points.

Dynamics of a fractal set of first-order magnetic phase transitions in frustrated Lu2CoMnO6

Magnetic frustration can produce exotic spin configurations and dynamics. Here, the authors explore the seemingly simple competition between ferromagnetic nearest- and antiferromagnetic next-nearest neighbor interactions along Ising spin chains, as realized in the magnetoelectric compound Lu${}_{2}$CoMnO${}_{6}$. Commonly, this situation is described by the axial next-nearest-neighbor Ising model. For the first time, the authors measure and calculate its correlated magnetic dynamics, resulting from a characteristic fractal set of first-order phase boundaries. Experiments and Monte Carlo simulations reveal a dynamics slowdown while approaching the phase transition regime.

Exploring dense baryonic matter and multi-strangeness at J-PARC

J-PARC is one of the world's highest-intensity proton synchrotron facilities for material and life sciences, neutrino physics, hadron and nuclear physics. By acceleration of heavy-ion beams, J-PARC could also become a high-intensity frontier heavy-ion beam facility (J-PARC-HI). For heavy-ion acceleration, we will build a heavy-ion linac and a booster ring as a compact heavy-ion injector, while we utilize the existing RCS and MR synchrotrons to accelerate 1011 Hz heavy-ion beams at 1–12 AGeV/c. We aim at exploring phase structures of the QCD phase diagram in a high baryon-density regime such as the first-order phase boundary and the QCD critical point, and also aim to search for multi-strangeness systems. We are developing the experimental design for a large acceptance spectrometer. In this work, physics feasibility of J-PARC-HI such as flow and hypernucleus measurements will be shown. Also, a plan to transport heavy-ion beams in the primary proton beamline at Hadron Experimental Facility and an upgrade plan to study heavy-ion collisions at the p+A experiment J-PARC E16 will be presented.

Enhancement and reentrance of spin triplet superconductivity in UTe2 under pressure.

Spin triplet superconductivity in the Kondo lattice UTe2 appears to be associated with spin fluctuations originating from incipient ferromagnetic order. Here we show clear evidence of twofold enhancement of superconductivity under pressure, which discontinuously transitions to magnetic order, likely of ferromagnetic nature, at higher pressures. The application of a magnetic field tunes the system back across a first-order phase boundary. Straddling this phase boundary, we find another example of reentrant superconductivity in UTe2. As the superconductivity and magnetism exist on two opposite sides of the first-order phase boundary, our results indicate other microscopic mechanisms may be playing a role in stabilizing spin triplet superconductivity in addition to spin fluctuations associated with magnetism.

First-order phase boundaries of the massive ( 1+1 )-dimensional Nambu–Jona-Lasinio model with isospin

The massive two-dimensional Nambu--Jona-Lasinio model with isospin (isoNJL) is reconsidered in the large $N_c$ limit. We continue the exploration of its phase diagram by constructing missing first-order phase boundaries. At zero temperature, a phase boundary in the plane of baryon and isospin chemical potentials separates the vacuum from a crystal phase. We derive it from the baryon spectrum of the isoNJL model which, in turn, is obtained via a numerical Hartree-Fock (HF) calculation. At finite temperature, a first-order phase boundary sheet is found using a thermal HF calculation. It interpolates smoothly between the zero temperature phase boundary and the perturbative sheet. The calculations remain tractable owing to the assumption that the charged pion condensate vanishes. In that case, most of the calculations can be done with methods developed in the past for solving the massive one-flavor NJL model.

Inhomogeneous chiral phases away from the chiral limit

The effect of explicit chiral-symmetry breaking on inhomogeneous chiral phases is studied within a Nambu–Jona-Lasinio model with nonzero current quark mass. Generalizing an earlier result obtained in the chiral limit, we show within a Ginzburg-Landau analysis that the critical endpoint of the first-order chiral phase boundary between two homogeneous phases gets replaced by a “pseudo-Lifshitz point” when the possibility of inhomogeneous order parameters is considered. Performing a stability analysis we also show that the unstable mode along the phase boundary is in the scalar but not in the pseudoscalar channel, suggesting that modulations which contain pseudoscalar condensates, like a generalized dual chiral density wave, are disfavored against purely scalar ones. Numerically we find that the inhomogeneous phase shrinks as one moves away from the chiral limit, but survives even at significantly large values of the current quark mass.

Field-induced phase coexistence in an artificial spin ice

Artificial spin-ice systems are magnetic metamaterials consisting of nanomagnet arrays that can be designed to study exotic magnetic states not found in natural materials. Typically, these arrays are modelled as interacting binary macrospins that can only be in an up or down state and are described by the Ising model. These materials have demonstrated ordering transitions, but only via a spontaneous symmetry-breaking mechanism. We have designed and studied a quadrupole artificial spin-ice system consisting of interacting plaquettes of coupled single-domain nanomagnets that can be interpreted as a composite, ternary variable. After annealing this system in an external magnetic field, we observe both a ferroquadrupolar and an antiferroquadrupolar phase, with an apparent first-order phase boundary and a coexistence regime. The phase diagram of this material is reminiscent of a model used to describe phase coexistence in the superfluid transition of 4He with 3He impurities. These results illustrate how composite magnetic objects realize exotic statistical physics models beyond the Ising model. Artificial spin-ice materials are usually described by spins that are either up or down. Here, a new type of spin ice is fabricated where the spins can be in one of three states with different coexisting phases separated by a first-order transition.

Thermodynamic investigation by heat capacity measurements of κ-type dimer-Mott organic compounds with chemical pressure tuning

Thermodynamic discussion on the physical properties around the Mott boundary of the dimer-Mott system of [Formula: see text]-type (BEDT-TTF)2X system, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene and X denotes monovalent counter anions is performed by heat capacity measurements of the [Formula: see text]-(d[n,n] BEDT-TTF)2Cu[N(CN)2]Br of which donor molecules are partially deuterated. We compare temperature dependences of heat capacity of d[2,2], d[3,3] and d[4,4] compounds (defined as shown in Fig. 1) obtained in rapidly cooled and slowly cooled conditions. While the heat capacity of d[2,2] compound shows cooling rate dependence with the slight change of the absolute values Cp between 25 K and 50 K, the temperature dependences of the other compounds do not give drastic change by changing the cooling rate. The compounds of d[2,2] and d[3,3] located very close to the Mott boundary show peculiar hump structure in the C[Formula: see text]T[Formula: see text] that is related to the existence of a kind of lattice instability region around the first-order phase boundary. The temperature and chemical pressure dependences of the anomalous region are discussed in terms of the phase diagram in which the Mott boundary has peculiar rounding at low-temperature region.

Reentrant Phase Diagram of Yb_{2}Ti_{2}O_{7} in a ⟨111⟩ Magnetic Field.

We present a magnetic phase diagram of rare-earth pyrochlore Yb_{2}Ti_{2}O_{7} in a ⟨111⟩ magnetic field. Using heat capacity, magnetization, and neutron scattering data, we show an unusual field dependence of a first-order phase boundary, wherein a small applied field increases the ordering temperature. The zero-field ground state has ferromagnetic domains, while the spins polarize along ⟨111⟩ above 0.65T. A classical MonteCarlo analysis of published Hamiltonians does account for the critical field in the low T limit. However, this analysis fails to account for the large bulge in the reentrant phase diagram, suggesting that either long-range interactions or quantum fluctuations govern low field properties.

Polyakov loop effects on the phase diagram in strong-coupling lattice QCD

We investigate the Polyakov loop effects on the QCD phase diagram by using the strong-coupling (1/g^2) expansion of the lattice QCD (SC-LQCD) with one species of unrooted staggered quark, including O}(1/g^4) effects. We take account of the effects of Polyakov loop fluctuations in Weiss mean-field approximation (MFA), and compare the results with those in the Haar-measure MFA (no fluctuation from the mean-field). The Polyakov loops strongly suppress the chiral transition temperature in the second-order/crossover region at small chemical potential, while they give a minor modification of the first-order phase boundary at larger chemical potential. The Polyakov loops also account for a drastic increase of the interaction measure near the chiral phase transition. The chiral and Polyakov loop susceptibilities have their peaks close to each other in the second-order/crossover region. In particular in Weiss MFA, there is no indication of the separated deconfinement transition boundary from the chiral phase boundary at any chemical potential. We discuss the interplay between the chiral and deconfinement dynamics via the bare quark mass dependence of susceptibilities.

Magnetic phase diagrams of Fe–Mn–Al alloy on the Bethe lattice

The magnetic behaviors of the Fe–Mn–Al alloy are simulated on the Bethe lattice by using a trimodal random bilinear exchange interaction (J) distribution in the Blume–Capel (BC) model. Ferromagnetic (J > 0) or antiferromagnetic (J < 0) bonds or dilution of the bonds (J = 0) are assumed between the atoms with some probabilities. It is found that the second- or the first-order phase boundaries separate the ferromagnetic (F), antiferromagnetic (AF), paramagnetic (P), or spin-glass (SG) phases from the possible other one. In addition to the tricritical points, the special points at which the second- and the first-order and the spin-glass phase lines meet are also found. Very rich phase diagrams in agreement with the literature are obtained.

Phase Competition, Solitons, and Domain Walls in Neutral–Ionic Transition Systems

Phase competition and excitations in the one-dimensional neutral-ionic transition systems are theoretically studied comprehensively. From the semiclassical treatment of the bosonized Hamiltonian, we examine the competition among the neutral (N), ferroelectric-ionic (I$_\mathrm{ferro}$) and paraelectric-ionic (I$_\mathrm{para}$) states. The phase transitions between them can become first-order when the fluctuation-induced higher-order commensurability potential is considered. In particular, the description of the first-order phase boundary between N and I$_\mathrm{ferro}$ enables us to analyze N-I$_\mathrm{ferro}$ domain walls. Soliton excitations in the three phases are described explicitly and their formation energies are evaluated across the phase boundaries. The characters of the soliton and domain-wall excitations are classified in terms of the topological charge and spin. The relevance to the experimental observations in the molecular neutral-ionic transition systems is discussed. We ascribe the pressure-induced crossover in tetrathiafulvalene-$p$-chloranil (TTF-CA) at a high-temperature region to that from the N to the I$_\mathrm{para}$ state, and discuss its consequence.

First-Order Melting of a Weak Spin-Orbit Mott Insulator into a Correlated Metal.

The electronic phase diagram of the weak spin-orbit Mott insulator (Sr(1-x)La(x))(3)Ir(2)O(7) is determined via an exhaustive experimental study. Upon doping electrons via La substitution, an immediate collapse in resistivity occurs along with a narrow regime of nanoscale phase separation comprised of antiferromagnetic, insulating regions and paramagnetic, metallic puddles persisting until x≈0.04. Continued electron doping results in an abrupt, first-order phase boundary where the Néel state is suppressed and a homogenous, correlated, metallic state appears with an enhanced spin susceptibility and local moments. As the metallic state is stabilized, a weak structural distortion develops and suggests a competing instability with the parent spin-orbit Mott state.

Solid−liquid critical behavior of water in nanopores

Nanoconfined liquid water can transform into low-dimensional ices whose crystalline structures are dissimilar to any bulk ices and whose melting point may significantly rise with reducing the pore size, as revealed by computer simulation and confirmed by experiment. One of the intriguing, and as yet unresolved, questions concerns the observation that the liquid water may transform into a low-dimensional ice either via a first-order phase change or without any discontinuity in thermodynamic and dynamic properties, which suggests the existence of solid-liquid critical points in this class of nanoconfined systems. Here we explore the phase behavior of a model of water in carbon nanotubes in the temperature-pressure-diameter space by molecular dynamics simulation and provide unambiguous evidence to support solid-liquid critical phenomena of nanoconfined water. Solid-liquid first-order phase boundaries are determined by tracing spontaneous phase separation at various temperatures. All of the boundaries eventually cease to exist at the critical points and there appear loci of response function maxima, or the Widom lines, extending to the supercritical region. The finite-size scaling analysis of the density distribution supports the presence of both first-order and continuous phase changes between solid and liquid. At around the Widom line, there are microscopic domains of two phases, and continuous solid-liquid phase changes occur in such a way that the domains of one phase grow and those of the other evanesce as the thermodynamic state departs from the Widom line.

Pressure–temperature phase diagram of CeAg

We have performed electrical-resistivity measurements under hydrostatic pressures ranging up to 4.47GPa on the cubic compound CeAg. A pressure–temperature phase diagram has been revised in a high-pressure range above ∼2.1GPa, where the system is known to enter into an unidentified phase through a first-order phase transition. We have found that the electrical resistivity shows a clear kink anomaly below ∼ 4K in this unknown phase. The anomaly shifts to lower temperature with increasing pressure, and shows a tendency to reach absolute zero at ∼6GPa, suggesting the presence of a pressure-induced quantum critical point around this pressure. We have also observed that a first-order phase boundary line, which separates the phase diagram into two regions of a tetragonal phase and the unidentified phase, seems to terminate in a critical end point located at (PCEP, TCEP) ∼(3.2GPa,200K). This suggests that the crystal symmetry of the unidentified phase above ∼2.1GPa is also tetragonal.

Completely packed O(n) loop models and their relation with exactly solved coloring models.

We investigate the completely packed O(n) loop model on the square lattice, and its generalization to an Eulerian graph model, which follows by including cubic vertices which connect the four incoming loop segments. This model includes crossing bonds as well. Our study was inspired by existing exact solutions of the so-called coloring model due to Schultz and Perk [Phys. Rev. Lett. 46, 629 (1981)], which is shown to be equivalent with our generalized loop model. We explore the physical properties and the phase diagram of this model by means of transfer-matrix calculations and finite-size scaling. The exact results, which include seven one-dimensional branches in the parameter space of our generalized loop model, are compared to our numerical results. The results for the phase behavior also extend to parts of the parameter space beyond the exactly solved subspaces. One of the exactly solved branches describes the case of nonintersecting loops and was already known to correspond with the ordering transition of the Potts model. Another exactly solved branch, describing a model with nonintersecting loops and cubic vertices, corresponds with a first-order, Ising-like phase transition for n>2. For 1<n<2, this branch is interpreted in terms of a low-temperature O(n) phase with corner-cubic anisotropy. For n>2 this branch is the locus of a first-order phase boundary between a phase with a hard-square, lattice-gas-like ordering and a phase dominated by cubic vertices. A mean-field argument explains the first-order nature of this transition.